Here is my way of doing it. It works best. I've been surprised by all the trouble people were having when doing it another way..
There's another solution too if you don't want to use litecom: open up the icon file in Preview, when it opens, press Command + C on your keyboard. Quit preview. Select the disk who's icon you want to change and Command + I, click the original icon (top left) in the window that opens, a blue glow effect should then appear. Press Command+V on your keyboard. Your icon is changed.